2.1 PALA
PALA is a compound which was initially developed as a transition state analogue inhibitor of aspartate transcarbamylase. Stark et al. (1974) J. Biol. Chem. 246:6599. Subsequently, PALA (NSC No. 224131) was thoroughly studied as an anti-cancer agent. See, for example, Johnson et al. (1976) Cancer Res. 36:2720-2725; Erlichman et al. (1982) J. Nat. Cancer Inst. 68:227-231.
PALA, its salts and analogues, and the preparation thereof are described in U.S. Pat. Nos. 4,179,464, 4,215,070, 4,267,126, 4,348,522 and British Patent Nos. GB 2008118 and GB 2051070 to Schultz et al. as well as in U.S. Pat. Nos. 4,154,759 and 4,178,306 to Parsons et al.
N-(phosphonoacetyl)-L-aspartic acid (PALA) inhibits de novo pyrimidine biosynthesis by blocking the enzyme, L-aspartic acid transcarbamylase (ATCase)--this enzyme catalyzes the condensation of L-aspartate and carbamyl phosphate the condensation of which is essential in the synthesis of orotic acid and the end product, uridine. The ultimate result of the inhibition is depletion of nucleotide pools viz UTP, CTP as well as nucleotide intermediates, viz, UDP-GlcN, CMP-NeuNAc which are essential for elongation of the oligosaccharide chain(s). Johnson, R. K., Aeon, T., Golden, A. and Stark, G. R. (1985) J. Med. Chem. 28:2720-262. Clinical Brochure, NSC No. 224131, Div. of Cancer Treatment, National Cancer Institute, Bethesda, Md. 1977. Thus, while PALA acts primarily by inhibiting nucleotide biosynthesis, the effect on its intermediates is also reflected in its end products: carbohydrates, proteins, as well as nucleic acids (RNA and DNA).
PALA exerts its action as a competitive inhibitor of carbamyl phosphate and as a non-competitive inhibitor of aspartate. Hooengraad, N.J. (1974) Arch. Biochem. Biophys. 161:76-82. Its Ka is 1000.times. more avid than that of the natural substrate, carbamyl phosphate. Moore, E. C., Friedman, J., Valdivieso, M., Plunkett, W. Marti, J. R. et al. (1982) Biochem Pharmacol. 31:3317-3321. Because of its relative lack of toxicity and the sensitivity of several solid murine tumors lines, PALA has been used in experimental oncology studies. Recent studies have shown that PALA possesses unique modulatory activity when used in combination with halogenated pyrimidines, e.g., 5-fluorouracil, both in vitro and in vivo. Liang, C., Donchower, R. C. Chabner, B. A. (1982) Mol. Pharmacol. 21:224-230; Ardalan, B., Galzer, R. I. Kenslet, T. W. et al. (1981) Biochem. Pharmacol. 30:2045-2049; Anakarahanonta, T., Holstege, A. and Keppler, D. O. R. (1980) Eur. J. Cancer 16:1171-1180. Most recently, PALA has been disclosed as a pyrimidine biosynthesis inhibitor which is useful for the treatment of autoimmune diseases, chronic inflammatory diseases, and of organ transplantation rejections. International Application PCT/US90/05942, published May 16, 1991 as WO91/06863.
A review of the literature has shown that PALA, when used alone for the treatment of cancer (neoplasia), is relatively nonefficacious in the clinical setting. Valdivies, M., Moore, E. C., Burgess, A. M., Marti, J. R., Russ, J., Plunkett, W. (1980) Cancer Treat. Rep. 46:1301-1305; Ehrichman, C., Strong J. M., Wiernik, P. H., McAvoy, L. M., Cohen, M. H. Levine, A. S., Hubbard, S. M., and Chabner, B. (1979) Cancer Res. 39:3992-3995; Grem, J. L. King, S. A., O'Dwyer, P. J. and Leyland-Jones, B. (1988) Cancer Res. 46:4411-4454.
A report on structure-activity relations involving phosphonoacetic acid ("PAA") and its analogs has appeared (Mao, J. C. H. et al. 1985 Antimicrob. Agents Chemother. 27(2):197-202). These workers found that PAA was a selective antiherpesvirus agent and that derivatization of PAA resulted in lower activity without exception. Specifically, PALA was found to be markedly less effective than the parent PAA by a factor of over 200.